The explosive deployment of wired and wireless communication infrastructure has recently enabled many novel applications and sparked new research problems. One of the unsolved issues in today’s Internet – the main topic of this thesis – is the goal of increasing data transfer speeds of end hosts by aggregating and simultaneously using multiple network interfaces. This objective is most interesting when the Internet is accessible through several, relatively slow and variable (typically wireless) networks, which are unable to single-handedly provide the required data rate for resource-intensive applications, such as bulk file transfers and high-definition multimedia streaming.

Communication devices equipped with multiple network interfaces are now commonplace. Smartphones and laptops are often shipped with built-in network adapters of different wireless technologies, typically enabling them to connect to wireless local area networks and cellular data networks (such as WLAN and HSPA). At the same time, wireless network coverage has become so widespread that mobile devices are often located in overlapping coverage areas of independent access networks. However, even if multiple interfaces are successfully connected to the Internet, operating systems typically use only a single default interface for data transmission, leaving secondary interfaces idle. This technical restriction is based on the fact that the majority of current Internet traffic is conveyed by transport protocols (TCP and UDP) that do not support multiple IP addresses per endpoint. Another crucial factor is that path heterogeneity introduces packet reordering, which can negatively affect the performance of transport protocols and strain the buffer requirements of applications.

This thesis explores the problem of multipath aggregation with the attempt to find solutions that achieve increased data throughput by concurrently utilizing heterogeneous and dynamic access networks. We strive for approaches that support existing applications without requiring significant modifications to the current infrastructure. The exploration starts at the IP layer with a study of the IP packet reordering that is caused by the use of heterogeneous paths. Our practical experiments confirm that multipath reordering exceeds the typical packet reordering in the Internet by an extent that renders the usual reordering metric useless. The outcome of our network-layer study is a proxy-based, adaptive multipath scheduler that is able to mitigate packet reordering while transparently forwarding a single transport connection over multiple paths.

After introducing a novel metric for quantifying the benefit of path aggregation, our analysis continues on the transport layer, where we investigate TCP’s resilience to multipath-inflicted packet reordering. While IP packet reordering is known for its destructive effect on TCP’s performance, our practical experiments indicate that a modern implementation is significantly more robust to packet reordering than standard TCP and achieves a substantial aggregation benefit even in certain cases of extreme multipath heterogeneity. In addition, we run a large set of emulation-based multipath experiments and identify several TCP parameters that lead to improved multipath performance when correctly tuned.

Finally, we present an application-layer solution that builds upon the idea of logical file segmentation for streaming a single video to multihomed clients. The novelty of this approach lies in diverting standard protocol features (i.e., HTTP pipelining and range retrieval requests) from their intended purpose and using them for scheduling video segments over different paths. Interoperable with existing server infrastructure, our proposed solution can be deployed in a lightweight and purely client-based manner. We validate the proposed algorithms by implementing them into an existing video streaming platform.

Media Performance Group

The interaction with digital media pervades most people everyday lives, both at home and at work. A person may easily understand that he interacts with digital media, or be completely oblivious to it. Application ideas appear limitless, yet so appear inhibitors of new uses. Inhibitors range from unchangeable laws of physics through scalability problems faced in processing digital media to a lack of understanding for people's requirements to appreciate a particular application.

The Media department investigates the means of overcoming or evading inhibitors for the use of time-dependent digital media in distributed systems. The department finds solutions by exploring, understanding and improving on a particular inhibitor in an application context. Improvements are found in better operating systems mechanisms, programming tools, protocols, distributed architectures, digital media formats or a better understanding of people's perception of media in a context. The department takes a multimedia systems approach, whereby successful research leads to quantifiable improvements and success is proven experimentally. The essential results of the research are algorithms, methods, tools or prototypes that provide solutions for overcoming a particular set of challenges in using time-dependent digital media in distributed systems that are practical and realistic today or in the near future.